Aluminosilicates and borosilicates constitute crystalline modifications of silicon dioxide wherein silicon atoms have been replaced by aluminium or boron atoms, respectively. The Si/Al and Si/B ratios of these porous materials may vary within very wide limits but, for use as a catalyst, the Si/Al ratio in an aluminosilicate catalyst generally ranges between about 6 and about 2000, while the Si/B ratio in a borosilicate catalyst generally ranges between about 3 and about 2000. Hereinafter the terms "aluminosilicate" and "borosilicate" will be denoted by the terms "zeolite" and "boralite", respectively.
Zeolites are generally known from the literature, U.S. Pat. No. 3,702,886 being pointed out in particular and herein incorporated by reference. Boralites have likewise been described, for example in U.S. Pat. Nos. 4,269,813 and 4,254,297 both of which are also incorporated by reference.
Silicon atoms in boralites and zeolites can be replaced by germanium atoms. Minor quantities of the silicon and/or germanium can in turn be replaced by other elements such as iron, chromium, vanadium, molybdenum, arsenic, manganese, or gallium.
Zeolites and boralites are characterized by their crystal structure, which is, in turn, an essential factor in the structure of their internal pore system. The internal pore system plays a large part in the catalytic properties of these materials.
Catalytic active sites will however not only occur in the internal pores of the material, but also on the external surface of the silicate crystallites. It should be emphasized that in this context as well as throughout this specification the expression `pore` is ment to refer to the structural, internal, pore system of the silicate materials involved, whereas the expression `surface` or `external surface` of the catalyst materials and/or crystallites is ment to refer to that surface of the materials which is not within said structural, internal pores.
As there exist catalytically active sites on the external surface of the silicate cristallites, there will also occur, of course, active sites near the entrances to the pores. Consequently, when these silicates are used as catalysts in a process such as hydrocarbon conversion, carbon will build up at the pore entrances and result in the pores closing off completely, or becoming less and less accessible as time passes to the entry of reactants into, or the exit of products out of, the pores. The catalytic activity of the zeolite or boralite accordingly decreases and the carbon buildup must be removed. The removal is laborious and expensive, in addition to which the yield of product decreases strongly as the buildup at the pore entrances progresses.
Known processes exist which have dealt with this problem in various fashions. For example, from the DD Pat. No. 111 091 it is known to block the active sites on the external surfaces of zeolite catalysts with alkaline compounds the molecular size of which exceeds that of the pores of the zeolite.
From U.S. Pat. No. 3,404,192 it is known to poison the external surface of zeolite catalysts with substances not capable of penetrating into the pores of the zeolite.
From U.S. Pat. No. 4,273,753 it is known to treat zeolites with dealuminating agents containing halogen. If the molecular size of the dealuminating agent is greater than the pore openings, substantially only the external surfaces of the zeolites will be dealuminated.
The disadvantage of these known processes is that the deactivating agent has to be very specific in view of the pore system of the material to be superficially, deactivated. In particular it is necessary, in order to avoid the partial deactivation of the pores themselves, there must be a substantial difference in size between the molecules of the reagents used for deactivating the external surface and the openings of the pores. The use of such very specific deactivating agents is not only detrimental to the universatillity of the method but also to the economy, and simplicity of use of the method. The further disadvantage of these known processes lies in the fact that they are limited to the treatment of catalysts having relatively small pores.